1. Field of the Invention
The present invention relates to an ultrasonic endoscope apparatus for viewing a body cavity, more particularly to an improvement of the construction of a distal end of an insertion portion having an ultrasonic vibrating element and an observation window.
2. Description of the Related Art
Several kinds of ultrasonic endoscope apparatuses having a probe of which is inserted in a body cavity to carry out a precise ultrasonic diagnosis of the state of the cavity have been proposed. U.S. Pat. Nos. 3,817,089 and 4,674,515 and Japanese Unexamined Patent Publication Kokai Sho 61-168,337, for example, discloses an ultrasonic endoscope apparatus in which a rotatable rotor directs an ultrasonic beam emitted from an ultrasonic vibrating element to carry out the so-called radial scan in a plane perpendicular to the center axis of the probe, the circumference of the rotor being covered with a distal end cap having an ultrasonic transmission window, and a side view or an oblique forward view observation window being provided in a side face of a rear end portion of the distal end cap.
U.S. Pat. No. 4,327,738 discloses an ultrasonic endoscope apparatus in which an electronic linear scan type ultrasonic vibrating unit is provided in a distal end of a probe in parallel with the central axis of the probe, and an optical viewing system having an observation window at the front thereof is provided at the rear of the vibrating unit.
Nevertheless, although the former endoscope apparatus can obtain a laminagram image over a broad scope of, for example 360 degrees, when the probe is inserted in a body cavity, since the field of view in front of the optical observation window of the probe is not fully covered, the probe may damage a mucous membrane or a diseased portion of the body cavity if a narrow portion or a portion at which bleeding easily occurs, such as a varix, exists in the body cavity. Further, when the operator attempts to insert the probe into the depths of an internal organ having many bent tubes, such as the large intestine, it is not possible to determine the direction in which the probe should be inserted, and thus the insertion of the probe becomes difficult.
On the other hand, although a front view is easily obtained in the latter endoscope apparatus when the probe is inserted, if the length of the distal end portion of the probe is too long, the probe can not be easily inserted into bent internal organs, and thus the length of a hard end portion of the probe is limited. Therefore, the length of the ultrasonic vibrating un provided in the hard end portion of the probe is limited to 3 cm, and therefore, a problem arises in that an ultrasonic laminagram image having only a narrow breadth of 3 cm can be obtained. Further, only an ultrasonic laminagram image in parallel with the axis of the probe can be obtained, and therefore, in order to ensure that a diseased portion in an internal organ is fully observed, the distal end of the probe must be inserted in the organ while rotating the probe through 360 degrees, and thus a diagnosis requires much time and the patient is subjected to much pain.